The present invention relates to a photothermographic recording material with very fine uniformly distributed silver halide particles.
Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
3. Imagewise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element containing other of the ingredients necessary for the chemical or physical process followed by uniform heating to bring about the changes in colour or optical density.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
Thermographic materials of type 1 can be rendered photothermographic by incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
U.S. Pat. No. 3,457,075 discloses a sheet material useful in imaging by a process involving exposure to a light-image followed by uniform heating and including a stratum containing (a) photosensitive silver halide catalyst-forming means and (b) heat-sensitive reactant image-forming means including (1) a water-insoluble silver salt of a long chain fatty acid as an oxidizing agent, and (2) a reducing agent for silver ion, the oxidation-reduction reaction of which to produce a visible change is accelerated by the catalyst; the stratum being further characterized in that a sufficient quantity, of at least about one-fourth mol percent based on the fatty acid silver salt, of the photosensitive means is in cataytic proximity with a sufficient proportion of the heat-sensitive means to provide a gamma infinity value of at least about 0.5 when the stratum is exposed image-wise to the light-image and the image is then developed by uniform heating and wherein the manufacture method for the sheet material comprises mixing with the fatty acid silver salt a source of halide ions under conditions permitting reaction therebetween with formation of the silver halide catalyst-forming means. As sources of halide ions for the conversion of organic silver salts to silver halides GB 1,547,326 lists: inorganic halides, halogen-containing metal complexes, onium halides (for example quaternary ammonium halides, quaternary phosphonium halides and ternary sulfonium halides), halogenated hydrocarbons, N-halo compounds and other halogen-containing compounds.
The standard teaching over such photothermographic materials based on a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the organic silver salt and a reducing agent for the organic silver salt is that the organic silver salt is formed, optionally in the presence of ex situ formed silver halide, in an aqueous medium and is precipitated and dried before dispersion in an organic solvent medium from which the dispersion is coated, the silver halide either being prepared ex situ, and either added to a dispersion of the organic silver salt as described in U.S. Pat. No. 3.080,254 or being present during the formation of the organic silver salt as disclosed in U.S. Pat. No. 3,839,049, or being prepared in situ from the organic silver salt by reaction with a halide ion source as disclosed in U.S. Pat. No. 3,457,075. In the latter case reaction of organic silver salt with a halide ion source, which can be inorganic or organic, occurs after the dispersion of the organic silver salt in a solvent medium and hence the reaction takes place in a non-aqueous medium. Photothermographic materials using silver behenate/silver halide dispersions produced by forming silver behenate in the presence of 50 nm particles of silver halide as disclosed in U.S. Pat. No. 3,839,049 require about 10 mol % of silver halide for optimal photosensitivity.
This production method is very inefficient as the organic silver salt after formation in water has to be separated and dried before dispersion in a solvent medium, is environmentally unsound as evaporation of solvent takes place during the coating process and it involves lengthy utilization of plant during the preparation of the organic silver salt dispersion and coating requires costly plant due to the need for solvent explosion prevention measures and solvent recovery to prevent solvent emission to the environment.
Furthermore, it is desirable spectrally to sensitize photosensitive silver halide in aqueous media as this permits the use of a broader range of spectrally sensitizing dyes.
Despite forty years of continuous research in this area, a production method for photothermographic materials based on a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the organic silver salt and a reducing agent for the organic silver salt which dispenses with these disadvantages of the current teaching, has to our knowledge not yet been developed.
WO 94/16361 discloses a multilayer heat-sensitive material for direct thermal recording for which no intermediate drying of organic noble metal salts is necessary and which is coatable from aqueous dispersions. This material comprises: a colour-forming amount of finely divided, solid colourless noble metal or iron salt of an organic acid distributed in a carrier composition: a color-developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a colour-forming reaction with the noble metal or iron salt; and an image toning agent: characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the colour-forming layer. However, this material suffers from poor stability both before and after image formation.
A further desirable aim is to reduce the amount of photosensitive silver halide necessary to achieve the required photosensitivity in photothermographic materials based on a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the organic silver salt and a reducing agent for the organic silver salt. This would increase the intrinsic stability of such materials and hence their photosensitivity (or the amount of silver halide necessary to attain the same photosensitivity), since this reduces the amount of stabilizer necessary to achieve acceptable pre- and post-exposure stability and hence the degree of stabilizer-induced photo-sensitivity loss.
It is therefore a first object of the invention to provide a photothermographic recording material comprising a photo-addressable thermally developable element for a photothermographic material with excellent image-forming properties.
It is therefore a second object of the invention to provide a photothermographic recording material comprising a photo-addressable thermally developable element based on.a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the organic silver salt and a reducing agent for the organic silver salt, which is produceable without necessitating intermediate drying of the organic silver salt.
It is therefore another object of the invention to provide a photothermographic recording material comprising a photo-addressable thermally developable element based on a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the organic silver salt and a reducing agent for the organic silver salt, which is coatable from an aqueous medium.
It is a further object of the invention to provide a photothermographic recording material comprising a photo-addressable thermally developable element based on a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the organic silver salt and a reducing agent for the organic silver salt requiring lower quantities of silver halide to achieve an acceptable photosensitivity.
It is a still further object of the invention to provide a recording process for a thermographic recording material with the above improved characteristics.
Further objects and advantages of the invention will become apparent from the description hereinafter.
Despite the general rule in silver halide photography that photosensitivity of a photographic material increases with increasing size of the silver halide crystals (as disclosed for example by A. P. H. Trivelli and W. F. Smith in 1939 in Photographic Journal, volume 79 in papers beginning on pages 330, 463 and 609 and in 1940 in Photographic Journal, volume 80 in a paper beginning on page 361 as cited on page 100 of xe2x80x9cThe Theory of the Photographic Processxe2x80x9d, Fourth Edition, edited by T. H. James and published in 1977 by Macmillan Publishing Co., Inc., New York) it has been surprisingly found that the photosensitivity of photothermographic materials of the present invention has been found to increase with decreasing silver halide crystal size at least down to diameters of 10 nm as shown by the reduced quantity of silver halide needed to obtain an acceptable image density under the same exposure and development conditions.
According to the present invention a photothermographic recording material is provided comprising a photo-addressable thermally developable element comprising a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the substantially light-insensitive organic silver salt and a reducing agent in thermal working relationship with the substantially light-insensitive organic silver salt and a binder, characterized in that the binder comprises a water soluble polymer, a water-dispersible polymer or a mixture of a water soluble polymer and a water-dispersible polymer and particles of the photosensitive silver halide which are non-aggregating in the photo-addressable thermally developable element and are uniformly distributed over and between particles of the substantially light-insensitive organic silver salt, at least 80% by number of the particles having a diameter, determined by transmission electron microscopy, of xe2x89xa640 nm.
According to the present invention a recording process is also provided comprising the steps of: (i) providing a photothermographic recording material comprising a photo-addressable thermally developable element comprising a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the substantially light-insensitive organic silver salt and a reducing agent in thermal working relationship with the substantially light-insensitive organic silver salt and a binder; (ii) bringing the recording material into the proximity of a source of actinic radiation to which it is sensitive; (iii) image-wise exposing the recording material to the actinic radiation; (iv) bringing the image-wise exposed recording material into proximity with a heat source; (v) thermally developing the image-wise exposed recording material; and (vi) removing the thermally developed image-wise exposed recording material from the heat source, characterized in that the binder comprises a water soluble polymer, a water-dispersible polymer or a mixture of a water soluble polymer and a water-dispersible polymer and particles of the photosensitive silver halide are non-aggregating in the photo-addressable thermally developable element and are uniformly distributed over and between particles of the substantially light-insensitive organic silver salt, at least 80% by number of the particles having a diameter, determined by transmission electron microscopy, of xe2x89xa640 nm.